It is desirable to measure the physical and chemical properties of metallic materials in order to ensure that such materials meet specifications to which they have been designed. In galvanizing steel wire, for example, it is desirable to ensure that the amount of the galvanizing material used to coat the wire is correct both for economic and operating reasons.
Galvanizing a wire comprises pulling the annealed and acid pickled wire through a bath of molten zinc and drawing the wire upward through a wiper to remove the excess zinc coating. To change the amount of coating, one can vary the speed at which the wire is drawn through the bath or the amount of wiping.
The measurement of the zinc coating is accomplished by removing samples of wire from the beginning and the end of the wire which was coated. The samples are then submitted to "gravimetric testing" wherein they are initially weighed and subsequently immersed in hydrochloric acid to remove the zinc coating. The samples are again weighed to determine the difference and, hence, the weight of the coating. Such results, while determining with some accuracy the weight of the zinc on the sample pieces, are slowly obtained and the sample pieces are only indicative of the amount of coating on the wire as a whole. Furthermore, it is a relatively expensive process.
Other techniques have also been used to measure coatings. One such technique is disclosed in U.S. Pat. No. 4,593,244 to Summers et al. This technique uses the so called "skin effect" to measure the amount of coating on a substrate. The aforementioned skin effect is so named because when a frequency is applied to a metallic substance, the magnetic flux is principally restricted to the outer portion of the body and a less permeable coating is more deeply penetrated than a more permeable substrate. Thus, if an oscillator driven sensor coil has a coated substrate positioned within the coil, the impedance will be different from its value when an uncoated substrate is placed within the coil. The use of the coating "hides" the substrate since the coating will affect the coil's impedance more than the substrate. Thus, the characteristics of the coating are indicated by the change in coil impedance which is readily measured.
In such a process, the oscillator frequency applied to the coil is important to the sensitivity of the instrument. While it is believed the explanations given throughout this application correctly explain the phenomena, such explanations are given in the interest of full and complete disclosure and applicant would not wish to be bound by the explanations if, subsequently, the explanations are found to be incorrect or partially so or if further explanations more accurately define the phenomena which are not presently known to the applicant.
There are two competing frequency dependant parameters to consider as follows: ##EQU1## As given above in (1), the first parameter that governs impedance sensitivity is the skin depth. The skin depth measured varies with the reciprocal of the square root of the frequency. Thus, in terms of skin depth criteria, the ideal applied frequency would produce a skin depth as deep as the coating at its thickest point. If a shallower skin depth were chosen, the instrument would "miss" part of the coating so that, essentially, the skin depth sets an upper limit on measurable coating thickness. If a deeper skin depth were chosen, an unnecessary amount of the substrate would be "seen" by the instrument and the coating measurement would become overshadowed by the spurious effects of the substrate. Thus, the skin depth criterion affects sensitivity in two ways. First, it sets the maximum measurable coating thickness and, second, it determines the amount of overshadowing from the substrate.
As given above in (2), the second parameter that governs sensitivity is the extent to which the coil resistance varies with the amount of the inserted sample coating. A large variation in resistance is easy to measure and leads to a sensitive instrument. The resistance of the coil and sample core combination increases with the square root of frequency. At least to the frequency upper limit of relation (2), increasing frequency increases the spread of the resistances produced by sample cores with varying amounts of coating. Thus, the resistance criterion also affects sensitivity in two ways. First, it sets the minimum measurable amount of coating and, second, it sets the minimum measurable change in amount of coating.
There is, in addition, a further important factor in the measurement system, namely the resonant frequency of the sensor coil and associated capacitance. Signals are strongest at resonance and thus for greatest sensitivity, a coil and capacitance should be used which has a resonant frequency at the oscillator frequency that was chosen by utilizing the criteria of relations (1) and (2).
The principle shortcomings of the previous attempts have been the complexity of the circuitry required and the imprecision of results stemming from insensitivity. Thus, only a fairly limited range of sample sizes could be measured. All three problems have arisen from the choice of either the applied or the resonant frequency, the frequencies chosen erring on the low side.
In one previous technique attributable to the present inventor in his undergraduate thesis entitled "Continuous On-line Electronic Zinc Coating Weight Measurement System for Galvanized Steel Wire" filed at Lakehead University on Apr. 28, 1988, a lower applied frequency was used than the resonance of the sensor coil and associated capacitance. Since the signal strength was also lower than necessary, changes in the resistance of the coil were difficult to measure and the implementation instrumentation was relatively insensitive. In choosing the applied frequency, the relationship set out in relation (2) was then not apparent. As a result, a lower frequency was used with the concomitant problems set out above.
In a further previous technique as disclosed in aforementioned Summers et al. reference, a skin depth margin of error was included. A low enough frequency was used to ensure that the skin depth was two to four times the predicted maximum coating thickness. This tradeoff resulted in dramatically compressing the coil resistance spread predicted by relation (2). This lead to unnecessarily complicated bridge and amplification circuitry. The result was that only coarse measurements could be made since small changes in the amount of coating did not vary the resistance of the sensor by a sufficiently measurable amount.